To those familiar with the art of metal can fabrication, it is well known that an inside protective coating is not only desirable, but many times, necessary, depending on the end use of the metal can. Thus, metal cans destined for beverage or food packing are always coated. There are many methods presently in use to effect a coating on the inside surfaces of metal cans. All the present methods of internally coating metal cans have attendant difficulties. Perhaps the greatest of these, by today's standards, is that attendant upon the use of a solvent-based coating material. The need for removal of the solvent following coating application causes a problem. The solvent must be disposed of without causing pollution. Efforts have been made to eliminate solvents from coating materials. Water based coatings have been used. Powder coatings have been used, generally applied by electrostatic means. None of these methods are entirely satisfactory. A powder coating material containing virtually no solvents that could be applied uniformly at high line speeds to inside metal can surfaces would be an ideal solution to this coating problem.
Two general methods are now commercially used in attempting to attain a uniform internal coating using powder material and an electrostatic process.
In the first of these, the powder particles are charged in an intense electrostatic field and then presented to the metal can, which attracts these particles by reason of its opposite charge. Here, the difficulties encountered range from extreme non-uniformity with an excess of the powder clinging to the first available metal surface, such as the rim of the can, to a slow process requiring great attention to controlled air flow transport of the charged powder particles.
In the second of these, the powder particles are charged as they issue, by air transport, from the end of a lance which moves within the metal can to effect the coating. Here, the difficulties arise from both non-uniformity and slow process speed. Once again, in this process, the metal can is of opposite charge to that of the particles, thereby causing the powder to be attracted to its surface.
Among published prior work of others, the U.S. Pat. of Gustin, No. 2,253,562, issued Jan. 16, 1951, teaches a technique for electrostatically coating the inside of a fluorescent glass lighting tube. It does involve a delayed application of electrostatic forces--i.e., electrostatic forces are not applied until after a cloud of powdered coating material is already dispersed within the tube. However, it is also clear that the powder is not in an ionized status when it is being distributed within the tube.
Rather, a distinct contrast to the subject invention, non-ionized powdered material is dispersed within the tube and thereafter an electrical potential difference is caused to exist between the hot glass tube (which is an electrical conductor at high temperatures) and an internal central pointed electrode 56. As the point 57 of this electrode is slowly withdrawn along the longitudinal axis of the tube, the powdered coating material is both ionized and then electrostatically attracted to the outer surfaces of the tube in the vicinity of the pointed end 57 of the electrode 56.
Gustin, U.S. Pat. 3,323,489, issued May 22, 1967, teaches an adaptation of the earlier Gustin patented method, this time for coating the inside of glass bulbs.
Other prior U.S. patents of general interest with respect to electrostatic coating of the inside of glass structures, metal structures and/or, generally, the application of powdered particles to surfaces by use of electrostatic forces include:
______________________________________ Pat. No. Patentee Issue Date ______________________________________ 2,811,131 Lopenski et al Oct. 29, 1957 3,690,298 Venturi May 22, 1970 3,904,930 Waldron et al Sept. 9, 1975 ______________________________________